GSM and PT unit system sets the mass‑to‑thickness framework for paper and paperboard used in consumer packaging, if substrates change density or coating load. GSM states mass per square meter and aligns with shipping weight, while PT states caliper in thousandths of an inch and aligns with stiffness limits in folding cartons. GSM values describe how papers range from 30–400 g/m² across newsprint, office grades, and boards, and PT values describe how boards fall between 8–16 pt for most packaging inserts. Density controls the GSM‑to‑PT link; if coatings or fiber bulk shift mass faster or slower than the thickness. Conversions use thickness and density to map GSM into PT or PT into GSM through equations that produce tables for densities such as 0.7, 0.8, and 1.0 g/cm³. Packaging specifications depend on both units because grammage fixes coating mass, die‑cut pressure, and pallet load, while caliper fixes fold radius, bending rigidity, and machinery clearance. Material properties, coating levels, fiber structure, and converting pressure set the variability in GSM‑to‑PT relationships, if each sheet compresses or expands during processing.
- What is Grams Per Square Meter (GSM)?
- What is the Point (Pt)?
- What are the Differences Between the Grams Per Square Meter (GSM) and Point (Pt)?
- How Does GSM Relate to Paper Thickness?
- How to Convert the Gram Per Square Meter (GSM) into Point (Pt)?
- How to Convert the Point (Pt) into Gram Per Square Meter (GSM)?
- How are GSM and Pt Used in Packaging Specifications?
- What Factors Affect GSM‑to‑PT and PT‑to‑GSM Conversions?
What is Grams Per Square Meter (GSM)?
GSM (grams per square meter) is the mass of paper or board expressed as grams per square meter (g/m²). It is the standard metric used to define basis weight in most regions. GSM indicates the amount of material present over a unit area, making it a primary indicator of material consumption and shipping weight. It is also widely used to classify and grade paper and board for printing and converting applications. Typical GSM ranges include light web and newsprint grades at 30–60 g/m² (such as newsprint and tissue liners), office and book papers at 70–120 g/m² (including copier and offset papers), and card stocks and boards at 120–400+ g/m² (such as folding carton and litho board). GSM is measured directly by cutting a representative sample, conditioning it under standard temperature and humidity, weighing it using a laboratory balance, and calculating the mass per square meter according to standardized testing procedures.
What is the Point (Pt)?
The point (pt) caliper denotes the thickness of a paper or paperboard sheet and expresses height in thousandths of an inch. One point equals 0.001 inch (1 pt = 0.001 in = 25.4 µm), and converters in North America use this unit to classify folding cartons, inserts, and paperboard grades that fall between 8 pt and 16 pt in most packaging runs. Thickness measurement relies on mechanical micrometers or non‑contact laser gauges pressed under controlled pressure, so the reading reflects true caliper instead of compression artifacts. Consistent and standardized measurement practices limit variability from creases, coatings, compressibility, and surface roughness if the sheet contains dense coatings or bulky pulp. A common production example appears in a 350 GSM C2S board that measures around 16 pt and in a 200 GSM pulp stock that measures around 13 pt.
What are the Differences Between the Grams Per Square Meter (GSM) and Point (Pt)?
The differences between the Grams Per Square Meter (GSM) and Point (Pt) include how each unit defines a separate physical property used in paper and packaging specifications. The table below lists the primary differences between GSM and pt so readers understand where each unit applies during material selection for consumer goods packaging.
| Comparison Category | GSM (Grams per Square Meter) | PT (Point Unit System) |
|---|---|---|
| Measured Property | Mass per square meter affects material weight and density | Thickness in thousandths of an inch affects caliper and rigidity |
| Main Use in Packaging | Evaluates material load, coating mass, and freight weight for items such as labels or linerboard | Evaluates mechanical strength and stiffness for items such as folding cartons |
| Direct Physical Meaning | Represents sheet mass, if surface coatings or fillers increase weight | Represents the heat caliper, if compression from calendering changes the thickness |
| Typical Ranges | 30–400 g/m² for papers, liners, and boards | 8–16 pt for packaging boards and inserts |
| How Stock Type Affects Values | Coated and filled grades increase mass without equal increases in thickness | Bulky pulps increase thickness without equal increases in mass |
| Approximate Cross-Mapping Examples | 200 g/m² pulp stock, 270 g/m² uncoated, 350 g/m² coated | ≈13 pt pulp stock, ≈14 pt uncoated, ≈16 pt coated |
| What Changes the Value | Fillers, coatings, lamination, and moisture shift GSM | Calendering pressure, fiber bulk, and surface roughness shift pt |
This table clarifies how GSM and pt diverge in function, measurement, and application so material selectors can compare mass-driven and thickness-driven performance during packaging development.
How Does GSM Relate to Paper Thickness?
GSM relates to paper thickness because grammage interacts with apparent density, which sets the caliper a sheet develops under standard pressure. A higher GSM raises thickness only if density stays constant; coated or filled grades shift density upward, so mass rises faster than caliper. Low‑density pulp stocks create bulkier sheets at the same GSM, which yields higher point values as seen in ranges such as 200 g/m² at roughly 13 pt for pulp stock, while dense C2S or SBS at 350 g/m² compresses into about 16 pt. This relation matters in packaging because caliper dictates stiffness and fold radius while GSM determines mass, shipping load, and coating content; designers compare both properties to match sheet strength with equipment limits.
How to Convert the Gram Per Square Meter (GSM) into Point (Pt)?
To convert the GSM into pt, compute pt = (GSM ÷ ρ) ÷ 25.4, if ρ expresses the apparent density of the sheet. This formula clarifies the caliper values shown in the table below because each GSM entry converts to thickness (µm) and then to points using the same ratio. The table below converts common GSM values to thickness and points for three representative apparent densities (0.7, 0.8, 1.0 g/cm³).
| GSM (g/m²) | T @ ρ=0.7 (µm) / pt | T @ ρ=0.8 (µm) / pt | T @ ρ=1.0 (µm) / pt |
|---|---|---|---|
| 60 | 85.7 µm / 3.37 pt | 75.0 µm / 2.95 pt | 60.0 µm / 2.36 pt |
| 80 | 114.3 µm / 4.50 pt | 100.0 µm / 3.94 pt | 80.0 µm / 3.15 pt |
| 100 | 142.9 µm / 5.62 pt | 125.0 µm / 4.92 pt | 100.0 µm / 3.94 pt |
| 150 | 214.3 µm / 8.43 pt | 187.5 µm / 7.38 pt | 150.0 µm / 5.91 pt |
| 200 | 285.7 µm / 11.24 pt | 250.0 µm / 9.84 pt | 200.0 µm / 7.87 pt |
| 300 | 428.6 µm / 16.87 pt | 375.0 µm / 14.76 pt | 300.0 µm / 11.81 pt |
The table clarifies how density reshapes point values: low-density pulp stock creates higher pt at the same GSM, while coated SBS at higher density produces lower pt for the same grammage.
How to Convert the Point (Pt) into Gram Per Square Meter (GSM)?
To convert the Point (Pt) into Gram Per Square Meter (GSM), the conversion uses thickness and apparent density to compute mass per unit area. GSM = (T × ρ) where T equals thickness in micrometers and ρ equals apparent density in g/cm³. One point equals 0.001 inch, which equals 25.4 µm, so the full expression becomes GSM = (pt × 25.4) × ρ. The formula produces a grammage value that matches the mass‑driven specifications for board, labels, or litho‑laminated stock.
The table below lists common pt values and their corresponding GSM values across three representative density ranges (0.7, 0.8, and 1.0 g/cm³).
| PT (points) | GSM @ ρ=0.7 | GSM @ ρ=0.8 | GSM @ ρ=1.0 |
|---|---|---|---|
| 10 pt | 177.8 g/m² | 203.2 g/m² | 254.0 g/m² |
| 12 pt | 213.4 g/m² | 243.8 g/m² | 304.8 g/m² |
| 14 pt | 248.9 g/m² | 284.5 g/m² | 355.6 g/m² |
| 16 pt | 284.5 g/m² | 325.1 g/m² | 406.4 g/m² |
The values above show how low‑density pulp stock yields lower GSM at a fixed pt, while coated SBS at high density produces the highest grammage. The table supports packaging decisions where thickness is fixed by machinery or carton design, but mass changes with material family.
How are GSM and Pt Used in Packaging Specifications?
GSM and PT define mass and caliper values that set print behavior, stiffness, coating load, freight weight, and machine compatibility in packaging specifications. GSM fixes material mass per unit area for items such as labels, liners, and carton stock; this mass determines coating thickness, ink holdout, die‑cut pressure, and pallet load, if the sheet density shifts with fillers or coatings. PT defines caliper in thousandths of an inch for folding cartons and inserts; this thickness determines bending rigidity, crease depth, and converting limits, if fiber bulk increases or compression from calendering reduces sheet height. Converters map standard grades across both systems to keep structural and mass targets aligned. Examples include 200 g/m² pulp stock at about 13 pt for light cartons, 270 g/m² uncoated stock at about 14 pt for matte folding work, and 350 g/m² coated SBS at about 16 pt for higher print fidelity. These values anchor carton design because machinery tolerances depend on caliper, while logistics calculations depend on grammage.
What Factors Affect GSM‑to‑PT and PT‑to‑GSM Conversions?
Factors that affect GSM‑to‑PT and PT‑to‑GSM conversions include how material density, fiber structure, and surface treatments shift the relationship between mass and caliper in packaging grades. The factors below show how each physical change alters grammage‑to‑thickness ratios.
- Density variation changes mass‑per‑thickness ratios in pulp stocks, coated boards, and filled sheets; low‑density bulk raises pt at the same GSM, if fiber structure traps more air.
- Fiber composition shifts caliper formation in mechanical pulps, chemical pulps, and recycled blends; long‑fiber sheets generate higher thickness at equal GSM if refining pressure stays moderate.
- Surface coatings add mineral mass on C2S or SBS grades; coating layers raise GSM faster than pt, if calendering compresses coating films during finishing.
- Fillers and additives increase sheet weight in clay‑filled or PCC‑filled stocks; fillers raise grammage at small increments of caliper, if retention remains high.
- Calendering pressure compresses surface height in fine papers and coated boards; heavy calendering reduces pt while GSM stays constant, if moisture stays stable.
- Moisture content shifts both mass and thickness during conditioning; higher moisture expands caliper and raises GSM marginally, if ambient humidity exceeds standard lab settings.
- Multilayer construction changes mass‑to‑thickness ratios in duplex or triplex boards; outer ply coatings add GSM while inner plies add bulk, if each ply uses different fiber grades.
- Compression during converting modifies the caliper in die‑cut or scored blanks; crease formation reduces pt without altering GSM, if scoring depth exceeds recommended limits.
- Stock classification groups uncoated, coated, and pulp‑heavy boards with distinct GSM‑to‑PT mappings; examples include 200 GSM pulp at about 13 pt, 270 GSM uncoated at around 14 pt, and 350 GSM coated SBS at about 16 pt, if density aligns with typical values.